What are the areas of construction unique to wooden playhouses?

Main Door Hinge Lines
Hinges on the entrance door can, and often do, create a gap when the door is partially or fully open, into which a child's finger might fit. This gap will trap and crush fingers, as the door closes. BS5665 advises that a gap of more than 12mm should be left along the hinge line to prevent this entrapment.
A 12mm gap, however, will allow the elements into the playhouse and it is suggested that a strong but flexible piece of material be fastened over the gap, (possibly on both inside and outside of hinge) to prevent ingress of rain, etc.
An alternative and more satisfactory answer would appear to be the use of a 'piano' hinge which would run the full height of the door, thereby satisfying both the above requirements. This can however leave a small v-shaped entrapment on the inside of the door.

Door Locks/Fastenings
The use of any locking/fastening device on the door would not comply with clause 3.2.2.6. of BS 4665, for although some latches do have internal handles and levers which an adult or older child could actuate, a younger child may be unable to operate them, particularly when in a distressed state. Magnetic catches would appear to be the only 'safe' and satisfactory method of ensuring the door will stay closed, yet allow younger occupants to escape by simply pushing the door open. The door must open with a maximum force of 50 Newtons, which is a force of approximately 5kg.

Stable Doors
The use of horizontally split stable doors is increasingly common in wooden playhouses, and can create a 'scissor' type of hazard between the two halves, when one half is being closed. If you wish to use this type of door, a gap of over 12mm will need to be created between the two halves of the door, with some kind of flexible cover over the gap, to ensure the door is weather-proof when closed.

Upper Floors
The revised EN71 (Draft of the new Toy Standard) will require floors more than 600mm above the main floor, to have a barrier or rail. Any gaps in the rail should be less than 90mm and greater than 12mm. The Bunkbeds Standard BS EN747-1 states a gap of 60-75mm is appropriate, for gaps in the rail etc. of the upper bunk and it would therefore seem a reasonable guideline to use, considering the similar hazards. The Bunkbed Standard also stipulates that the ladder access gap in the barrier should be between 300-400mm, and again, it would seem reasonable to use this guide to ensure the guard rail(s) only have a gap big enough to allow access to the ladder an no more. The minimum height of the barrier is hard to stipulate as each playhouse will vary in the amount of available height above the upper surface. But, it should be more than a token rail, and should be securely constructed and fastened to the playhouse main structure.

Ladders
The Revised Toys Standard stipulates that any opening in a frame, situated more than 600mm above the ground, must be either less than 90mm (and greater than 12mm) or more than 230mm and the Bunkbed Standard stipulates a distance between successive treads on a ladder or between 200mm and 300mm, with a usable tread width of at least 300mm. It would therefore be reasonable to manufacture a ladder with a gap of 230-300mm between the rungs, with a rung width of at least 300mm. Rungs too close or far apart could cause a child to miss one and fall from the ladder.
The issue of hand rails is again difficult to give definitive advice on and it will be up to individual manufacturers to decide to use them or not. A vertical ladder may be better without a rail as a child would normally hold on to the rungs. Any rail used must be secure.

Finger/Hand Entrapments
There should not be any gaps or holes in the wooden structure in which children might get their fingers or hands trapped.

Knots and Splinters
'Wood shall not display any insect holes and knots shall not be loose.' 'The surfaces and accessible edges of wooden toys shall not have splinters.' BS 5665 Part 1 - Clause 3.1.2.

Nails/Screws
The pointed ends of screws, nails and similar fastenings used in the manufacture of toys shall not be accessible …….' (Clause 3.2.1.3) - screws or nails should not protrude from the wood - at either end. Screw heads should not have sharp burrs on them.

Windows
Plain annealed glass must not be used - acrylic panels would appear to be the safest and cheapest option. Do not put strips of adhesive lead on either the inside or outside of the panes to avoid the possibility of lead poisoning.

Coatings
Any paint, varnish or preservative used must comply with BS 5665 Part 3 in that levels of Lead or Mercury etc. must be low.

Instructions/Warnings
Like any toy, playhouses MUST be accompanied by and the name and address of the manufacturer and any instructions or warnings you think necessary for the safe use and maintenance of the playhouse.

Age Restrictions
Most manufacturers accept that wooden playhouses are not suitable for the very young - like those under 3 years of age - BUT, some of the larger more elaborate playhouse with upper floors etc. are clearly not suitable, even for slightly older children. It is up to YOU as the manufacturer to specify an age at which you think a child will be able to cope with the special challenges your playhouse will pose to a child at play.

General Strength
Playhouses should be strong enough to withstand the normal behaviour of children at play. Upper floors in particular must be more than strong enough to take the weight one or more children who could fit on it.